1. Field of the Invention
The present invention relates to a method of plasma-etching a target layer comprising silicon oxide, and a process gas used in the method.
2. Description of the Related Art
With an increase in the degree of integration level of semiconductor devices, such techniques as can etch a target object accurately in accordance with a design have become more important. An error between a designed dimension and an etched dimension is used as a criterion in etching and referred to as a technical term, critical dimension loss (CD loss).
For example, in these days, it is necessary to form a contact hole having a diameter of 0.3 .mu.m and a high aspect ratio in a silicon-oxide-based layer, such as an SiO.sub.2 film, which is widely used as an interlevel insulating film. In this case, if an etching selection ratio of the silicon oxide film to a photoresist is low, a little CD loss is generated due to retreatment of the photoresist, so that the yield is greatly affected.
On the other hand, with an increase in the diameter of target substrates to be processed, such as semiconductor wafers, etching apparatuses for processing the substrates one by one have become the main current, in order to perform a uniform and fine process. The etching apparatuses of the above described one-by-one type require a high etching rate, so that a high throughput is obtained. In order to meet this demand, the etching apparatuses currently used are constituted such that plasma of a high density is generated in a process chamber, thereby ensuring a high etching rate.
Gases based on CxFy are generally used as an etching gas for plasma-etching a silicon-oxide-based layer, such as an SiO.sub.2 film. Among the CxFy-based gases, a gas, in which x:y=1:2, such as C.sub.4 F.sub.8 gas, is representative, since such a gas provides an excellent balance between a high etching selection ratio and a high etching rate. If the ratio between C and F in an etching gas is deviated from an optimum value, the etching ratio may be decreased, or the etching rate is decreased due to an amount of by-product deposition, such as carbide.
The C.sub.4 F.sub.8 gas is generally used under a low pressure of less than 10 mTorr, so that it is difficult to control the dissociation of the gas. As a result, problems are apt to occur such that fluorine-contained radicals are produced excessively so as to decrease the etching selection ratio, or fluorine-contained radicals are insufficient so as to cause a deposition to adhere and decrease the etching rate. Further, the etching rate and the aspect ratio presented by the C.sub.4 F.sub.8 gas are not satisfactory in consideration of a high throughput and a high degree of integration level currently required, even if the C.sub.4 F.sub.8 gas is accurately controlled over its dissociation. Furthermore, the C.sub.4 F.sub.8 gas is apt to generate a "sloping shoulder" at the opening of a contact hole when forming the contact hole in practice, even if the etching selection ratio of a silicon oxide film to a photoresist is set at a high value, thereby presenting a larger CD loss in reality.